Sample preparation and analysis presents many logistical problems. Conventionally, many medical samples (such as blood, saliva, urine and swab eluate) are provided to a doctor, for example a general practitioner doctor (GP) or a principle care physician (PCP), in a local surgery without the equipment necessary to analyse the sample. Hence, the sample must be sent to a laboratory where the sample is analysed. The test results must then be collated and returned to the GP to analyse the results and make a diagnosis. This approach is inadequate. Firstly, there is a significant risk that a sample is lost in transit or mismatched with the wrong patient. Moreover, whilst recent developments in technology have reduced the overall time taken to conduct the test, the delay involved in sending the sample to a laboratory is unsatisfactory.
Nevertheless, analytical systems of the kind found in laboratories are complex and it is often difficult to provide sufficient amounts of pure targets from source samples to reliably perform downstream analytical assays. This typically prohibits local GP surgeries from being able to carry out such tests on site.
However, in recent years efforts have been made to reduce the scale of the analytical systems to make tests faster and simpler to run, and require smaller quantities of sample. For instance, “laboratory on a chip” (LOC) devices (a subset of microfluidic devices) integrate almost all medical tests or diagnostic operations performed in a hospital on a single microfluidic chip. The channels forming such microfluidics devices handle small fluid volumes and are connected together so as to achieve a desired function such as mixing of a sample, moving the sample through the device, reacting the sample with different reagents, and so on. These chips may be inserted into machines to control the performance of a test and measure the results.
However, it has been found that handling a sample in a microfluidics device can be very difficult. In such small channels as are found on a conventional LOC, it is difficult to apply external forces to move the sample from one site to another to perform different actions on the sample. There is also a limit to the complexity of a LOC device which operates purely using capillary action. Furthermore, owing to the small sample sizes of LOC's, the devices have reduced sensitivity and the probability of a target being present in the sample is thus reduced.
An alternative approach is to use a fluidic cartridge. The scale of the components of a fluidic cartridge is larger than for a microfluidic device, and so it becomes possible to move a sample through various different sites to perform different actions on it. This makes it possible to perform more complex tests than may be conducted using typical LOC devices, whilst still providing an analytical system of potential use in a local GP surgery.
Scientific assays useful in medical diagnostics have increasingly involved biochemical procedures, such as the polymerase chain reaction (“PCR”). The PCR assay has provided a powerful method of assaying for the presence of defined segments of nucleic acids. It is therefore desirable to perform a PCR assay on a fluidic cartridge.
Reducing PCR to the microchip level is important for portable detection technologies and high-throughput analytical systems. The method can be used to assay body fluids for the presence of nucleic acid specific for particular pathogens, such as the Chlamydia trachomatis bacterium, HIV or any other pathogenic microbe.
The introduction of commercially available automated DNA amplification assays has allowed more laboratories to introduce these technologies for routine testing of specimens. However, there is a need to improve the fluidic devices used for this purpose.
It is requirement of devices employing PCR technology in the analysis and processing of fluid samples that a well-defined volume of processed fluid may be delivered to detection chambers for analysis of the sample. It is particularly important in LOC type cartridges where multiple fluid sample preparation steps are performed within the cartridge that the tolerances of the channel and chambers in the sample processing region are not allowed to stack up prior to processing and analysis of the fluid sample as this may lead to fairly large errors in the calculated amount of fluid required to fill the PCR chamber and subsequently be delivered to the detection chambers.
In certain LOC type cartridges, reagents may be dried down in the PCR chamber. This means that over-filling the PCR chamber and allowing an excessive amount of fluid sample to flow past the sample processing region may lead to a lack of reagents remaining in the PCR chamber. Under-filling the PCR chamber by even a small amount may lead to air pockets in the PCR chamber which in turn may lead to unstable thermocycling. To ensure that the PCR chamber is completely filled, slight over-filling of the chamber may be desired. However, even slightly overfilling the PCR chamber leads to unprocessed fluid downstream of the PCR chamber which may then dilute processed fluid sample as it is moved from the PCR chamber to the detection chambers.
WO2009108260 discloses a method for delivering a fixed volume of fluid to a microfluidic device comprising configuring a device with a sample loop comprising a desired volume, wherein the sample loop is removable, using one or more pneumatically actuated valves on a microfluidic device to fill the sample loop with the fixed volume of the fluid and delivering the fluid to the microfluidic device. Thus, a specific volume of fluid in the sample loop can be injected into the main channel by closing the intermediate valve, opening the flow through valves (606 and 608) and applying pressure to the main channel. The sample loop and a pass-through microfluidic channel are fluidically connected at a first junction and a second junction, and wherein at least one junction comprises a T-valve.
GB200711618 discloses a process for determining the concentration of nucleic acids in a sample in a microfluidic device. The process includes introducing a sample into a first chamber, carrying out a number of cycles of an amplification reaction to be carried out in cycles for amplifying nucleic acids, transferring a defined volume which is a fraction of the volume of the first chamber, and which has amplified nucleic acids into a second chamber and replacing the transferred defined volume with fresh reagents for the amplification reactions. The volume transfer means may be a controllable metering pump, a reciprocating pump or a similar means which can be used to move a defined volume from the first chamber to the second chamber.